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[InstCombine] Optimize icmp-select-icmp

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In special cases select instructions can be eliminated by
replacing them with a cheaper bitwise operation even when the
select result is used outside its home block. The instances implemented
are patterns like
    %x=icmp.eq
    %y=select %x,%r, null
    %z=icmp.eq|neq %y, null
    br %z,true, false
==> %x=icmp.ne
    %y=icmp.eq %r,null
    %z=or %x,%y
    br %z,true,false
The optimization is integrated into the instruction
combiner and performed only when all uses of the select result can
be replaced by the select operand proper. For this dominator information
is used and dominance is now a required analysis pass in the combiner.
The optimization itself is iterative. The critical step is to replace the
select result with the non-constant select operand. So the select becomes
local and the combiner iteratively works out simpler code pattern and
eventually eliminates the select.

rdar://17853760



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218721 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Gerolf Hoflehner
2014-10-01 00:13:22 +00:00
parent 8f70c4827a
commit 2318c2f28d
5 changed files with 277 additions and 9 deletions
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8
lib/Transforms/InstCombine/InstructionCombining.cpp
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@@ -90,6 +90,7 @@ INITIALIZE_PASS_BEGIN(InstCombiner, "instcombine",
"Combine redundant instructions", false, false)
INITIALIZE_PASS_DEPENDENCY(AssumptionTracker)
INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfo)
INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
INITIALIZE_PASS_END(InstCombiner, "instcombine",
"Combine redundant instructions", false, false)
@@ -97,6 +98,8 @@ void InstCombiner::getAnalysisUsage(AnalysisUsage &AU) const {
AU.setPreservesCFG();
AU.addRequired<AssumptionTracker>();
AU.addRequired<TargetLibraryInfo>();
AU.addRequired<DominatorTreeWrapperPass>();
AU.addPreserved<DominatorTreeWrapperPass>();
}
@@ -2933,12 +2936,9 @@ bool InstCombiner::runOnFunction(Function &F) {
AT = &getAnalysis<AssumptionTracker>();
DataLayoutPass *DLP = getAnalysisIfAvailable<DataLayoutPass>();
DL = DLP ? &DLP->getDataLayout() : nullptr;
DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
TLI = &getAnalysis<TargetLibraryInfo>();
DominatorTreeWrapperPass *DTWP =
getAnalysisIfAvailable<DominatorTreeWrapperPass>();
DT = DTWP ? &DTWP->getDomTree() : nullptr;
// Minimizing size?
MinimizeSize = F.getAttributes().hasAttribute(AttributeSet::FunctionIndex,
Attribute::MinSize);